1. Field of the Invention
This invention relates to special receptacles or packages adapted to contain food and to receptacles used to cook food by means of a heat source. It also relates to packaging suitable for other, non-food purposes, when ease and economy of manufacture, thermal insulation, shape stability at high service or operating temperature, and heat and impact resistance are significant factors.
2. Description of the Related Art
Products in which food is packaged, prepared and served are ubiquitous in modem life. Ranging from preparation, pre-cooking, presentation, serving, cooking, storage and reheating, uses to which such products are put vary widely. Further, ranging from single-use disposable packaging, through inexpensive limited multi-use products, through durable goods, such containers vary widely in cost and durability. Manufactured from materials including laminated cardboard, thermoformed polymer, foamed polymer and injection molded polymer, such products are also of widely varying composition.
Examining uses for such products in more detail, in the United States in particular, significant industrial activity is devoted to producing single use, disposable packaging for serving food in the restaurant industry. Polystyrene foam boxes have been used for some time for food packaging as in restaurant take-out boxes. Laminated cardboard containers are also used extensively for packaging restaurant take-out food. The pizza restaurant industry is dependent to a large degree on the utility of cardboard pizza take-out boxes.
With the increasing use of prepared frozen and heat-and-serve foods, demand has multiplied for single use or limited multi-use packaging in which food is prepared, precooked and marketed by producers and then cooked and served by consumers. While frozen dinners, packaged and sold as such for later cooking by the consumer, such as claimed in U.S. Pat. No. 3,244,537, have long been a mainstay of the American table, packaged prepared meals stored on the shelf at room temperature for purchase and cooking by the consumer, such as described and claimed in U.S. Pat. No. 5,904,946 are becoming increasingly common as well.
More expensive, longer-lived plastic containers for food storage, such as depicted and claimed in U.S. design Pat. No. D277,632 or the well-known Tupperware® are useful and popular items in most American kitchens. Further, consumers have welcomed the recent availability of inexpensive limited multi-use containers for dishware, storage and cooking, such as Ziploc® brand storage containers with Snap 'n Seal lids, as depicted and claimed in U.S. design Pat. No. D476,861.
Despite the ubiquity and variety of such products, the prior art is subject to significant limitations. Few materials or packages even approach suitability for all the uses to which food packaging is subject. Some prior art packages, which serve their limited uses well, are too expensive for other uses. Others are completely incapable of use outside of their limited area of applicability. Further, there are some uses that are desirable for food packaging that are not met at all by the prior art. In addition, much prior art food packaging is recycled with difficulty, if at all, and therefore is environmentally costly.
As is well known to those in the art, polystyrene foam in the prior art is in two basic forms: thermoformed extruded sheet (ESF) and expanded bead foam (EBF).
ESF foamed polymers are created by foaming extrusion, which entails producing or forcing a non-reactive foaming gas into a molten polymer mixture or alternately creating gas with chemical reactions within the molten polymer, thereby forming bubbles in the melt. The foamed melted material is extruded as a sheet of plastic containing fine bubbles forming microcells and is then allowed to cool. For forming ESF objects, the ESF material is subsequently cured for a period of time to allow gas pressures in the microcells to become optimal for forming, after which the material is thermoformed.
Boxes made from ESF Polystyrene foam are adequate for transporting food and provide excellent thermal insulation, enabling consumers to keep restaurant food warm from purchase until later consumption. However, polystyrene foam in general has little tensile strength and containers tear easily. Further, because it has little heat resistance, polystyrene foam is unsuitable for more than brief warming in a microwave oven and is wholly unsuited to heating in a conventional oven. In addition, polystyrene foam containers generally do not seal well and so are unsatisfactory for extensive storage of food. Lastly, the geometry of objects that can be thermoformed with prior art ESF is quite limited, restricting height to depth ratios to considerably less than 1:1, and wall angles to no less than about 40 deg from vertical.
EBF foam is made by saturating polystyrene pellets with blowing agent (typically a hydrocarbon gas such as butane or pentane), followed by blowing steam through the pellets, which penetrates the beads and acts as a secondary blowing agent while it heats the beads sufficiently to cause foaming. In forming objects, the foamed beads are then transferred to a molding machine where they are compressed and further steamed causing the beads to fuse to make the desired object, such as a polystyrene cup.
EBF foamed objects may be formed with height to depth ratios over 1:1 and wall angles approaching vertical. They are produced in net shape, requiring no trimming after manufacture, and, like all polystyrene foam products, they provide excellent thermal insulation. However, EBF foamed products have the same drawbacks of polystyrene foam products generally, and, in addition, EBF foamed products have even less tensile strength and durability than ESF polystyrene foamed products.
Regardless of method of manufacture, prior art foamed packaging suffers from a number of additional significant drawbacks. Polystyrene foamed materials can't withstand high temperatures and therefore are not microwaveable or ovenable above, about the boiling point of water. In addition, such foam containers usually lack quality, tight fitting lids, reducing their utility for food storage. Finally, all prior art polystyrene foamed materials, regardless of method of manufacture, are non-biodegradable and have little value for recycling, and therefore are environmentally costly.
While coated cardboard containers may be microwaved and, in fact, if comprised of appropriate materials, may be used for limited cooking and reheating in conventional ovens, such containers are unsuitable for extensive cooking or for storage beyond very short term. Further, the insulation properties of coated cardboard are slight, and clearly inferior to those of polystyrene foam. Furthermore, because of non-degradable, non-recyclable laminations due to the plastic coatings required for significant food contact life, many such containers are scarcely more recyclable than polystyrene foam.
Prior art pizza boxes, generally composed of corrugated cardboard, serve to insulate and transport the food. Most are rectangular in shape and many are not dimensioned for heating in home microwave ovens, and the cardboard material is unsuitable for use in a conventional oven. Further, prior art pizza boxes are unsuitable for long-term storage, because they do not seal tightly, allowing pizza to become stale, and the cardboard material decomposes as it absorb oils and liquids from food contained therein. While some prior art has employed foamed polymer for pizza boxes, as in U.S. Pat. No. 4,848,543, such prior art comprises boxes principally of polystyrene foam. Not only is such foam subject to the general limitations of polystyrene foam containers noted above, but its low heat resistance makes it unsuitable for receiving pizza immediately after cooking.
While polymer containers for frozen and other pre-cooked foods are commonly microwavable, many such containers are not adaptable for cooking in conventional ovens. Some solid polymer containers, such as those of highly crystallized PET (CPET) are useable in both microwave and conventional ovens. Solid polypropylene containers can be heated to about the temperature of boiling water for sterilization, but cannot withstand oven cooking temperatures. Foil containers, while almost obsolete, are unusable in microwave ovens. Regardless of suitability for microwave or oven, all such containers generally provide little if any thermal insulation for foods and are therefore unsuitable for maintaining foods at serving temperature. In addition, such containers usually lack quality, tight fitting lids, reducing their utility for food storage. Further, both foil and solid polymer containers are considerably more costly to produce than foamed polymer containers.
Longer-lived containers of the Tupperware sort provide excellent food storage but provide little thermal insulation. Such containers are not heat resistant and so, while they may be used for fast heating in microwave ovens, they are unsuitable for longer cooking times and may not be used in conventional ovens. In addition, such containers cost many times the cost of other containers considered herein. While the newer, shorter-lived multi-use containers are considerably less expensive, they also suffer from shortcomings in insulation capability and heat resistance.
Yet a further limitation applicable to much of the prior art processes for production of food containers is that scrap material from the manufacture of such containers is generally of little value and, in fact, may require costly disposal. Accordingly, the configuration of prior art food containers is often constrained by the need to minimize the scrap produced in their manufacture, thereby resulting in containers that are less than optimally shaped for their purpose.
What is needed are food containers suitable for pre-cooking or sterilization, insulated transport, use as a cooking vessel, reheating, and storage of food. What is needed further is an economical way of producing such containers at low cost. What is yet further needed is a process for producing such containers whereby there is little economic constraint on container shape. What is yet further needed are such containers that are also environmentally sound.
It has recently been discovered that polymer foam articles may be produced on an industrial scale by a novel process, gas impregnated thermoforming (GIT). In U.S. Pat. No. 5,684,055 to Kumar et al., incorporated herein by reference in its entirety, a roll of polymer sheet is provided with a gas channeling means interleaved between the layers of polymer. The roll is exposed to a non-reacting gas at elevated pressure for a period of time sufficient to achieve a desired concentration of gas within the polymer. The saturated polymer sheet is then separated from the gas channeling means and bubble nucleation and growth is initiated by heating the polymer sheet. After foaming, bubble nucleation and growth is quenched by cooling the foamed polymer sheet. The foamed sheet may then be thermoformed.
As further elaborated in PCT patent application number PCT/US2004/015246, titled METHOD OF PRODUCING THERMOFORMED ARTICLES FROM GAS IMPREGNATED POLYMER, filed contemporaneously herewith and incorporated herein by reference, such process is suitable for foaming with a wide range of gas/polymer systems comprised of non-reacting gas and amorphous or semi- crystalline thermoplastic polymer materials, including C02 with polyethylene, polyethylene terephthalate(PET), polyvinyl chloride, acrylonitrile butadiene styrene, polycarbonate, and polypropylene, while N2 may be used with polystyrene.
In U.S. Pat. Nos. 5,223,545 and 5,182,307 to Kumar et al., both incorporated herein by reference in their entirety, PET is shown to have its crystallinity levels raised by saturation with high pressure CO2 gas. Furthermore it has been shown that the crystallizing gas remains in the polymer for a time in substantial quantities after foaming and enhances crystallization during thermoforming.
It has been further been discovered that objects made from such foams possess surprising qualities that render such objects particularly suited for food packaging applications. Based upon these discoveries, it is an object of this invention to provide food containers suitable for pre-cooking or sterilization, insulated transport, use as a cooking vessel, reheating, and storage of food. It is a further object of this invention to provide an economical way of producing such containers at low cost. It is yet a further object of this invention to provide food packaging that is more ecologically sound for more varied applications than is available in the prior art.